Terminal electrode compositions for multilayer ceramic capacitors

ABSTRACT

The present invention relates to terminal electrode compositions for multilayer ceramic capacitors. More specifically, it relates to terminal electrode compositions for multilayer ceramic capacitors, which compositions are made of a copper-based powder and an organic binder that can be fired at a low temperature in a nitrogen atmosphere.

FIELD OF THE INVENTION

The present invention relates to terminal electrode thick filmcompositions for multilayer ceramic capacitors. More specifically, itrelates to terminal electrode compositions for multilayer ceramiccapacitors (MLC), which compositions are made of a metal-based powderdispersed in an inert liquid organic medium, which is fireable at a lowtemperature in a neutral or reducing atmosphere.

BACKGROUND OF THE INVENTION

Electrically conductive pastes composed of a base metal such as copperor nickel dispersed with an inorganic binder and an organic binderdispersed in a solvent are widely used as terminal electrodecompositions for multilayer ceramic capacitors. These conductive pastecompositions are typically fired in a neutral or reducing atmosphere(such as nitrogen) to suppress oxidation of the constituent metal andinternal electrode. Accordingly, of the ingredients contained in thepaste, it is necessary for the organic binder which must be burned offin the firing step to have sufficient thermal decomposability in thistype of atmosphere. Various types of acrylic polymers are well suitedfor this purpose. For instance, JP-A 2001-307549 describes examples inwhich compounds such as methyl methacrylate, ethyl methacrylate andbutyl methacrylate are used.

However, prior-art electrically conductive pastes have the followingdrawbacks. The fired film must have a thickness which is sufficientlylow to satisfy the chip specifications and achieve good reliabilityduring mounting. For example, at chip dimensions of 1.2 mm (width)×1.2mm (thickness)×2.0 mm (length), a terminal film thickness of not morethan 90 microns is appropriate. The method ordinarily used to controlthe film thickness within this range is a step called “blotting,” inwhich excess paste is removed after coating. However, this increasescosts due to the increased manufacturing tact time it represents and theincrease in man-hours required to recycle the paste that is removed.

Approaches that have been tried in order to eliminate blotting includethat of optimizing the paste characteristics to enable the applicationof a thinner film. One such method has involved reducing the pasteviscosity so as to lower the amount of paste deposited to the elementassembly of the MLC. However, the low viscosity allows the paste to sagon the sidewalls, preventing the shape to be maintained.

Moreover, although film thickness-reducing effects can be expected withthe use of a method for lowering the inorganic solids content within thepaste, there is a corresponding increase in organic binder and thinnercomponents, which lowers the viscosity. Increasing the amount of organicbinder to check this decline in viscosity will ensure that a suitableviscosity is achieved, but an excessive increase in the amount oforganic binder per unit volume harms thermal decomposability of theorganic binder fired in a neutral or reducing atmosphere.

It is therefore an object of the invention to provide a thick film pastecomposition for terminal electrodes, which composition has a reducedsolids content yet maintains a suitable paste viscosity and is able toensure sufficient thermal decomposability of the organic binder.

DETAILED DESCRIPTION OF THE INVENTION

The conductive thick film paste composition of the invention comprises aspecified amount of a methyl methacrylate polymer having a suitablemolecular weight, the solids within the paste composition can be loweredto 75 wt % or less (e.g., 70 wt %), making it possible to reduce thefilm thickness and thus form the desired terminal electrodes. Theinvention is described more fully below.

The organic medium is preferably one prepared by the dissolution of anacrylic polymer within a suitable solvent. The polymer has a highthermal decomposability within a neutral or reducing atmosphere.

The polymer used in the invention is methyl methacrylate, but the methylmethacrylate may be combined with polymers selected from ethylmethacrylate and butyl methacrylate, and copolymers of acrylatecompounds, and blends of these listed above.

The ratio of inert medium to solids in the composition may varyconsiderably and depends upon the manner in which the dispersion of thesolids in medium is to be applied. Dispersion contains 45 to 76 wt %solids and 24 to 55 wt % medium.

Accordingly, the invention provides a terminal electrode composition formultilayer capacitors, which composition comprises 30 to 71 wt % of aconductive powder selected from copper powder, nickel powder andcopper-nickel alloy powder dispersed in an organic binder in solventsuch as CARBITOL® acetate and butyl CARBITOL® acetate (CARBITOL® is aregistered trademark of Union Carbide Chemicals & Plastics TechnologyCorporation), wherein the organic binder is composed of one or moretypes of methyl methacrylate (MMA) polymer dissolved in an organicsolvent, at least one of the methyl methacrylate polymers having anumber-average molecular weight of at least 100,000 and a weight-averagemolecular weight of at least 1,000,000, such that the methylmethacrylate polymer accounts for 2.0 to 9.0 wt % of the paste, based ontotal composition.

To remove the organic medium, suitable oxygen doping in the nitrogenfiring furnace is ordinarily carried out within a thermal decompositiontemperature range of 150 to 450° C. The methyl methacrylate polymercontent for carrying out sufficient thermal decomposition is not morethan 9.0 wt %, and preferably not more than 7.0 wt %, based on the totalpaste. At a methyl methacrylate polymer content of more than 9.0 wt %,the polymer content per unit volume becomes to high for the amount ofoxygen supplied. As a result, the level of unburned organic residuesrises, which can cause sintering defects.

The organic medium confers a viscosity which allows the paste to bedeposited on a substrate in an appropriate shape, and also maintains thestrength of the dried coating. The use of a methyl methacrylate polymeris especially preferable for achieving a sufficient dry coat strengtheven when the polymer is used in a small amount. The use of less than2.0 wt % results in an excessive decrease in the viscosity, preventing asufficient dry film strength.

In the practice of the invention, the base metal particles are selectedfrom a copper powder, a nickel powder or a copper-nickel alloy powder. Acopper powder is preferred. Copper powder particles are selected fromspherical or of indeterminate shape and have an average particle size of0.5 to 30 μm, and flake-like copper particles having a particle size of0.1 to 30 μm, and mixtures thereof. Base metal particles that are toolarge compromise the density of the terminal electrode producedtherefrom. On the other hand, if the particle size is too small, thedispersion properties differ from those of the organic medium, givingrise to a change in rheology which makes it difficult to achieve anideal coated shape.

The content of base metal particles within the paste is 30 to 71 wt %.Below this range, a dense sintered film is not obtained; whereas abovethis range, the desired paste viscosity is not achieved.

Illustrative, preferred, non-limiting examples of the glass frit used inthe present invention include those composed of Si—B—Ba glass, Si—B—Pbglass and Si—B—Zn glass. The softening point of the glass frit as aninorganic binder is closely associated with the firing temperature. Toohigh a softening point inhibits sintering, whereas too low a softeningpoint promotes sintering. The firing temperature of the paste accordingto the invention is about 700 to 950° C. Hence, when firing is carriedout at about 750° C., for example, to keep the composition fromundergoing an excessive degree of sintering yet allow it to achieve asuitable degree of density, it is preferable for the glass softeningpoint be set within a range of 500 to 650° C.

The paste composition has a glass frit content from 5 to 15 wt %, andpreferably from 8 to 12 wt %, based on the overall composition. When toolittle glass frit is added, a fired film having sufficient density toserve as a barrier to the plating solution cannot be obtained, andadhesion to the capacitor assembly is inadequate. On the other hand, theaddition of too much glass frit causes glass components to rise to thesurface of the fired film, greatly compromising the plating adhesion.The glass frit is preferably a finely divided powder having a particlesize of 0.5 to 20 μm, and especially 1 to 10 μm. Too large a particlesize results in a low density, whereas too small a particle size resultsin dispersion properties that differ from those of the organic binder,altering the rheology and making it difficult to achieve an ideal coatedshape.

In the practice of the invention, the above-described base metalparticles and glass frit are dispersed in an organic medium to form apaste composition. The composition is coated onto a terminalelectrode-forming site of a multilayer ceramic capacitor. It is thenfired at a temperature of 700 to 950° C. to form terminal electrodes.Nickel or solder plating is then applied as a soldering surface to theterminal electrodes after they have been fired, thereby giving finishedterminal electrodes.

By including within the paste 2.0 to 9.0 wt % of at least one type ofmethyl methacrylate polymer having a weight-average molecular weight ofat least 1,000,000, an electrically conductive paste is obtained thathas an adequate dry coat strength, is free of paste sagging at end faceseven at a low solids content, and has sufficient thermal decomposabilityof the organic components when fired in a neutral or reducingatmosphere, and is free of adverse effects upon sintering by unburnedorganic matter.

EXAMPLES

Examples of the invention and comparative examples are given below. Allpercentages are based on total composition.

Example 1

A methyl methacrylate polymer having a weight-average molecular weightof 1,200,000 (22 wt %) was dissolved in butyl CARBITOL® acetate (78 wt%) to form an organic medium. 30 wt % of the above organic medium, 63 wt% of spherical copper powder having an average particle size of 3 μm and7 wt % of Si—B—Ba glass frit were each weighed out in the indicatedamounts and uniformly dispersed by blending in a three-roll mill to forma paste.

The Si—B—Ba glass frit had the composition by weight:

-   -   35.0% BaO    -   23.1% B₂O₃    -   13.5% SrO    -   12.5% SiO₂    -   4.5% ZnO    -   3.7% MgO    -   2.4% Al₂O₃    -   2.3% Na₂O    -   1.2% SnO₂    -   1.0% TiO₂    -   0.4% K₂O    -   0.4% LiO

Example 2

15 wt % a methyl methacrylate polymer having a weight-average molecularweight of 1,200,000 (22 wt %) was dissolved in butyl CARBITOL® acetate(78 wt %), 10.5 wt % methyl methacrylate having a weight-averagemolecular weight of 200,000 dissolved in butyl CARBITOL® acetate formedthe organic vehicle, 67 wt % of spherical copper powder having anaverage particle size of 3 μm and 7.5 wt % of Si—B—Ba glass frit ofExample 1 were each weighed out in the indicated amounts and uniformlydispersed by blending in a three-roll mill to form a paste.

Example 3

15 wt % of a methyl methacrylate polymer having a weight-averagemolecular weight of 1,200,000 (22 wt %) was dissolved in butyl CARBITOL®acetate (78 wt %), 10.5 wt % methyl methacrylate-butyl methacrylatecopolymer having a weight-average molecular weight of 150,000 wasdissolved in butyl CARBITOL® acetate (78 wt %) to form an organicvehicle, 67 wt % of spherical copper powder having an average particlesize of 3 μm and 7.5 wt % of Si—B—Ba glass frit of Example 1 were eachweighed out in the indicated amounts and uniformly dispersed by blendingin a three-roll mill to form a paste.

Comparative Example 1

30 wt % butyl methacrylate polymer having a weight-average molecularweight of 680,000 (22 wt %) was dissolved in butyl CARBITOL® acetate (78wt %) to form an organic vehicle of the above organic vehicle, 63 wt %of spherical copper powder having an average particle size of 3 μm and 7wt % of Si—B—Ba glass frit of Example 1 were each weighed out n theindicated amounts and uniformly dispersed by blending in a three-rollmill to form a paste.

Comparative Example 2

30 wt % methyl methacrylate polymer having a weight-average molecularweight of 200,000 (22 wt %) was dissolved in butyl CARBITOL® acetate (78wt %) to form an organic vehicle, 63 wt % of spherical copper powderhaving an average particle size of 3 μm and 7 wt % of Si—B—Ba glass fritof Example 1 were each weighed out in the indicated amounts anduniformly dispersed by blending in a three-roll mill to form a paste.

Comparative Example 3

16.7 wt % methyl methacrylate polymer having a weight-average molecularweight of 200,000 (22 wt %) was dissolved in butyl CARBITOL® acetate (78wt %) to form an organic vehicle, 75 wt % of spherical copper powderhaving an average particle size of 3 μm and 8.3 wt % of Si—B—Ba glassfrit of Example 1 were each weighed out in the indicated amounts anduniformly dispersed by blending in a three-roll mill to form a paste.

Test Methods

Evaluation Tests:

The pastes formulated as described above were coated onto multilayerceramic capacitor chips with dimensions of 1.2 mm (width)×1.2 mm(thickness)×2.0 mm (length), fired at a temperature of 750° C. in anitrogen atmosphere to prepare test pieces, and the film thickness atthe ends were measured. In addition, the end faces of the coated chipswere examined for sag and rated as acceptable (OK) or unacceptable (NG).The paste compositions and evaluation results are shown below in Table1.

TABLE 1 EX 1 EX 2 EX 3 CE 1 CE 2 CE 3 Organic vehicle 1 Resin MMA MMAMMA — — — Weight-average ×10,000 120 120 120 — — — molecular weightPolymer amount % 22 22 22 0 0 0 Organic vehicle 2 Polymer MMA MMA/BMABMA MMA MMA Weight-average ×10,000 — 20 15 68 20 20 molecular weightPolymer amount % 0 22 22 22 22 22 Paste Organic vehicle 1 % 30 15 15 0 00 Organic vehicle 2 % 0 10.5 10.5 30 30 16.7 Glass % 7 7.5 7.5 7 7 8.3Copper powder % 63 67 67 63 63 75 Solids content % 70 74.5 74.5 70 7083.3 Polymer content % 6.6 5.61 5.61 6.6 6.6 3.674 Results Filmthickness μm 75 89 88 76 73 121 End face sag OK OK OK NG NG OK MMA:methyl methacrylate BMA: butyl methacrylate

It is apparent from the results in Table 1 that in Examples 1 to 3according to the invention, each of which includes a type and amount ofmethyl methacrylate which satisfies the conditions set forth in claim 1,sagging of the paste was not observed on the end faces (OK), and thefilm thickness was less than 90 microns. By contrast, in each ofComparative Examples 1 to 3, either the film thickness was greater than90 microns or sagging of the paste was observed on the end faces of thecoated chip (NG).

1. An electrically conductive paste fireable in a neutral or reducingatmosphere comprising (a) 30 to 71 wt % conductive powder being selectedfrom the group of copper powder, nickel powder and copper-nickel alloypowder and (b) an inorganic binder, both dispersed in an inert organicmedium; wherein the organic medium comprises at least one methylmethacrylate (MMA) polymer dissolved in solvent, said methylmethacrylate polymer having a number-average molecular weight of atleast 100,000 and a weight-average molecular weight of at least1,000,000, such that the methyl methacrylate polymer accounts for 2.0 to9.0 wt % of the paste, wherein the amount of the inorganic binder is inthe range from 5 to 15 wt %, wherein the inorganic binder is selectedfrom Si—B—Ba glass, Si—B—Pb glass, Si—B—Zn glass, or mixtures thereofand the conductive powder and inorganic binder combined is in the rangefrom 45.0 wt % to 76 wt %.
 2. The conductive paste of claim 1, whereinthe organic medium further comprises ethyl methacrylate, butylmethacrylate, copolymers of acrylate compounds, or mixtures thereof. 3.A method of forming a terminal electrode comprising: (a) forming theconductive paste of any one of claim 1 or claim 2 (b) coating thecomposition of (a) onto a terminal electrode-forming site of amultilayer capacitor; and (c) firing the multilayer capacitor in (b) toform a finished terminal electrode.